DARPA is testing a wind sensor that could allow devices in its Adaptable Lighter-Than-Air (ALTA) balloon program to spot wind speed and direction from a distance and then make the necessary adjustments to stay in one place. Stratospheric balloons have enormous potential to operate like low-cost satellites at the edge of space – providing communication in remote or disaster-hit areas, following hurricanes, and monitoring pollution – but can only stay in one area for a few days at a time. In the stratosphere – about 60,000 feet (18,300 meters) high – winds blow in different directions at different altitudes, so theoretically it should be possible to find a wind blowing in any desired direction simply by changing altitude.
“By flying higher we hope to take advantage of a larger range of winds,” says ALTA project manager Alex Walan.
ALTA will operate at 75,000 to 90,000 feet (22,900 to 27,400 meters) where the winds are even less predictable.
The wind sensor, called Strat-OAWL (short for “stratospheric optical autocovariance wind lidar”), is made by Ball Aerospace. OAWL shines pulses of laser light into the air, a small fraction of the beam is reflected back, and the reflected laser light is gathered by a telescope. By analyzing the doppler shift, OAWL can determine the speed and direction of the wind. OAWL is also capable of “looking” in two directions at once thus giving a better indication of wind speed and direction.
Previous versions of OAWL flown in aircraft have measured winds more than 14 kilometers (8.6 miles) away with an accuracy of better than a meter per second. The main challenge with Strat-OAWL has been shrinking it to fit the space, weight, and power requirements of the ALTA balloons. The ALTA test flight program has already begun, with flights lasting up to three days, and will continue with steadily longer flights.